general agent: add context pruner, tweak prompts #5546

Job	Run time
build-and-check	2m 26s
test-integration-packages/internal	1m 1s
test-integration-packages/billing	1m 4s
test-integration-sdk	39s
test-integration-cli	38s
test-integration-web	48s
test-integration-packages/agent-runtime	43s
test-packages/billing	42s
test-packages/agent-runtime	55s
test-common	42s
test-.agents	35s
test-cli	1m 7s
test-integration-common	39s
test-packages/internal	46s
test-sdk	43s
test-web	51s
	14m 19s

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